1. Technical Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus capable of performing parallel imaging (PI). In particular, the present invention relates to both magnetic resonance imaging systems and parallel imaging methods which reduce artifacts inherent to parallel imaging by improving a post-processing unfolding technique.
2. Related Art
Magnetic resonance imaging constructs images from MR signals emanated from an object in response to magnetically excited nuclear spins of the object in a static magnetic field at a Larmor frequency of the spin.
In the field of magnetic resonance imaging, study of fast imaging has become active. For example, a fast imaging technique called in general parallel imaging has been known, which requires a multiple RF coil consisting of a plurality of RF coils (i.e., element coils). This parallel imaging technique has historically been called “multicoil fast imaging technique,” “PPA (Partially Parallel Acquisition) technique,” and/or “subencoding technique.”
Parallel imaging can be performed in a variety of modes, which include (1) a technique of calculating data to be skipped in k-space; (2) a technique of unfolding data in real space (called a “subencoding technique” or a “SENSE technique”), and (3) a technique of combining Sum-Of-Square images (called a “PILS”-technique) formed from the subencoding or SENSE technique. Techniques belonging to mode (2) have been known by such references as “Ra J. B. and Rim C. Y., Fast Imaging Method Using Multiple Receiver Coils with Subencoding Data Sets, ISMRM p.1240, 10991”; “Ra J. B. and Rim C. Y., Fast Imaging Using Subencoding Data Sets From Multiple Detectors, MRM 30:142-145, 1993”; “Pruessman K. P., Weiger M., Scheidegger M. B., and Boesiger P., SENSE: Sensitivity Encoding for Fast MRI, MRM 42:952-962, 1999”; and “Japanese patent publication No. WO99/54746(1998)”.
Basically, these techniques for parallel imaging uses an array coil (hereinafter referred to as Phased Array Coil: PAC) composed of a plurality of RF coils (i.e., element coils), which is one kind of a multiple RF coil, and adopts subencoding acquisition. This subencoding acquisition requires that phase encoding be skipped every so many predetermined steps, so that the total number of phase encoding steps is reduced down to an amount equal to “1/the number of RF coils” from the predetermined number of phase encoding steps otherwise necessary for reconstructing an ordinary image (i.e., with no skipped phase encoding steps).
The respective RF coils receive echo signals in a concurrent manner. The echo signals received by each RF coil are used to produce a set of image data. Hence FOV (field of view) of each image, which is produced from echo signals acquired by each RF coil having a smaller individual FOV, whereby scan time is shortened for fast imaging.
However, such an image produced using echo signals acquired by each RF coil includes wrap-around or folding (which is also called “aliasing”) in edge regions of each image. In parallel imaging, the fact that plural RF coils have different sensitivity distributions is thus utilized to perform, as post-processing, unfolding to unfold each image acquired using each RF coil. This unfolding is processed based on the spatial sensitivity maps of the RF coils.
A plurality of pieces of images, which have been subjected to unfolding processing, are then combined into a final full-FOV image. Therefore, the parallel imaging technique is able to accelerate to a fast scan (i.e., fast imaging) and finally provide wide FOV-images covering, for example, the whole abdominal area of an object to be scanned.
However, as described above, in the case of the parallel imaging, it is theoretically inevitable that each image acquired using each RF coil is subjected to the folding phenomenon. There is a problem in that there remain artifacts due to the folding, which are known as back-folding, as long as the existence region of an object to be scanned is within FOV specified as one imaging parameter.
To be specific, conventional parallel imaging plans a scan, in which conditions of unfolding processing are decided according to a final image specified by an operator with the use of such parameters as ROI, matrix size, FOV given during an imaging plan. The final image is normally rectangular in the shape. The decided unfolding processing conditions are used to produce unfolded images. In order to perform the unfolding processing in a preferable manner, operators should specify the size of a ROI (region of interest) exactly including the existence region of each object to be scanned. However, in practical clinical examinations, exactly specifying such ROIs becomes a considerable burden on operators. It is thus easier for operators to roughly specify the ROIs, resulting in back-folding artifacts occasionally occuring and deteriorating image quality for interpretation.
With reference to FIGS. 1A to 1C, such conditions will now be detailed.
FIG. 1A illustrates one conventional example in which a ROI defining a region subjected to an MR scan is set in a proper manner. A ROI is given so as to completely cover an object to be scanned and imaging conditions are given through a scan plan, before the subencoding technique is applied to data acquisition from the object. In this example, the subencoding technique is performed with the number of phase encoding steps reduced to one half of that required for standard Fourier imaging, so that the reduction rate of data acquisition time is 2 (i.e., two-fold speed). The folding, in this example, will cause superposition at two positions between adjacent folded FOVs. Hence, applying the unfolding processing on the assumption that two positions between adjacent folded FOVs are merely superposed with each other provides an image with no folded FOVs.
On the other hand, FIG. 1B illustrates another conventional example in which a ROI for imaging is set improperly. To be specific, during a scan plan for setting various imaging parameters, the ROI has been placed to specify an FOV (corresponding to the size of a desired final image) smaller than the actual existence region of an object to be scanned. In this example, when performing a scan at the foregoing two-fold speed, the folding will cause, in part, superposition between two folded FOVs, that is, superposition of three positions belonging to the three FOVs. This folding cannot be removed even if the unfolding processing on the foregoing two-fold speed is executed.
FIG. 1C exemplifies a conventional plan carried out in the manner shown in FIG. 1B, but subjected to imaging other than parallel imaging. However, in the example shown in FIG. 1C, since folding is caused only at edges of the FOV, there occurs no serious diagnostic problem in observing the final image.
When making reference to only FIGS. 1A to 1C, it may seem that the foregoing problem can easily be removed by merely setting, during a scan plan, a rectangular FOV slightly larger than a final-image desired size. In practice, however, the elimination of the problem will not be so simple. A multi-slice imaging technique is frequently adopted in the actual clinical application, where a ROI on one or more slices other than a slice used for a scan plan may be set to be less than the existence region of an object. Namely the existence region is beyond the FOV given through the ROI that has been set, like FIG. 1B. Hence the current situation is that it is not easy to always set a proper ROI. FIG. 2 illustrates such an undesirable situation, in which one of the slices suffers from the fact that a ROI is given improperly.
In imaging the heart, scans are normally made along the minor axis, major axis, and/or sections in the four chambers. FIGS. 3A and 3B pictorially illustrate some types of imaging of the heart along the minor axes thereof. However, this imaging also frequently suffers the foregoing problem, because the sectional shapes themselves of the heart are complicated and sections differ largely patient by patient. In addition, the same problem may occur in imaging a baby in the womb, because it is required that a section to be scanned coincide with the baby's orientation.
Even for imaging a coronal image of the trunk, it appears to be a reality that it is difficult to plan a proper FOV completely including a desired trunk section, because the arms positioned by the trunk extend largely in the lateral direction.
As understood from the above, a proper scan plan for the parallel imaging technique is not always carried out.